Such injection valves must be able to dose fluids even in the case of high fuel pressure. One design to ensure this is the “free-lift” design. According to this design, the armature of the electro-magnetic actuator unit travels about a “pre-stroke gap” before it engages the needle to open the injector. Thus, kinetic energy is accumulated before the actual opening.
The “free-lift”-concept uses a so called free-lift spring that biases the armature away from an upper retaining element, which is fixed to the valve needle. When the actuator unit is energized, the armature travels against the load of this spring before engaging the upper retaining element for moving the valve needle. Generally, the further the armature travels before engaging the valve needle, the more kinetic energy it will have accumulated. Therefore, in order to manage high fuel pressures, a large spring travel is advantageous.
On the other hand, a compact injector is advantageous as space is tight in automotive applications. Therefore, one solution is to arrange a free-lift spring in a recess in the armature. This, however, reduces the guidance length of the valve needle and can cause functional problems and wear.